(a) Field of the Invention
The present invention relates to a positive active material for a lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. More particularly, the present invention relates to a positive active material for a lithium battery with excellent high-capacity and thermal stability, a method of preparing the same, and a rechargeable lithium battery including the same.
(b) Description of the Related Art
Lithium ion rechargeable batteries have been widely used as a power source for portable electronic device since 1991. Recently, electronic, communication, and computer industries have become remarkably developed to provide potable devices such as camcorders, cell phones, laptop computers, and so on. Therefore, the lithium ion batteries are required to supply the power for these portable electronic devices. Particularly, a lithium rechargeable battery used as a power source for a hybrid electric vehicle also having an internal combustion engine is being actively researched in the United States, Japan, Europe, and so on.
A high capacity battery for an electric vehicle is the initial stage of development. Generally, a nickel hydrogen battery is used due to the safety thereof, but the lithium ion battery is advantageous in terms of energy density. However, the lithium ion batteries still have problems of both a high price and safety to be solved. Particularly, both LiCoO2 and LiNiO2 positive active materials that are commercially available have an unstable crystal structure due to de-lithiation upon charging the battery, and thus the thermal characteristics are fatally deteriorated. That is, when the overcharged battery is heated to 200-270° C., the structure is remarkably deformed, and thus a reaction of emitting oxygen is carried out by deforming the structure in the lattice (J. R. Dahn et al., Solid State Ionics, 69, 265, 1994).
Concurrently commercially available small lithium ion rechargeable batteries generally include a positive active material of LiCoO2. LiCoO2 is a material having stable charge and discharge characteristics, high stability, and a smooth discharge voltage, but as Co is a rare material and its cost is high, and it is toxic to people, alternative positive electrode materials to replace Co are required. Although LiNiO2 having a layered structure similar to LiCoO2 has a high discharge capacity, it has not commercially developed due to its unstable thermal and life-cycle characteristics as well as its lack of safety at a high temperature. In order to solve these problems, it has been attempted to substitute some nickel with transition elements so that the exothermic temperature is increased and to make a broad exothermal peak such that a rapid exothermal reaction is inhibited (T. Ohzuku et al., J. Electrochem. Soc., 142, 4033, 1995, No. 9-237631). However, the results have not yet been confirmed.
In addition, LiNi1−xCoxO2 (x=0.1-0.3) materials in which some nickel is substituted with thermally stable cobalt shows good charge-discharge and cycle-life characteristics, but it cannot provide a thermally stable battery. A Li—Ni—Mn-based composite oxide in which some Ni is substituted with thermally stable Mn and a Li—Ni—Mn—Co-based composite oxide in which some Ni is substituted with Mn and Co and methods of preparing the same have been suggested.
For example, the Japanese Patent laid-open Publication Hei 08-171910 discloses a method of preparing a positive active material of LiNixMn1−xO2 (0.7≦x≦0.95) including mixing a solution of Mn-included salt and Ni-included salt with an alkaline solution to co-precipitate Mn and Ni, mixing the co-precipitation with a lithium hydroxide, and firing the same.
Recently, Japanese Patent laid-open Publication No. 2000-227858 disclosed a positive active material in which Mn and Ni compounds were uniformly distributed at an atomic level to provide a solid solution instead of the transition elements being partially substituted into LiNiO2 or LiMnO2. However, according to European Patent No. 0918041 or U.S. Pat. No. 6,040,090, although LiNi1−xCoxMnyO2 (0≦y≦0.3) has improved thermal stability compared to that of materials composed of only Ni and Co, the reactivity of Ni4+ with the electrolyte solution sets a limit on commercialization. In addition, European Patent No. 0872450 A1 discloses LiaCobMncMdNi1−(b+c+d)O2 (M=B, Al, Si, Fe, Cr, Cu, Zn, W, Ti, or Ga) in which Ni was substituted with another metal as well as Co and Mn. However, as the active materials disclosed in these patents still include Ni, the thermal stability of the active material is not fully satisfied.
The most spotlighted alternatives to LiCoO2 may include Li[Ni1/2Mn1/2]O2 and Li[Ni1/3CO1/3Mn1/3]O2 having a layered structure in which nickel-manganese and nickel-cobalt-manganese are mixed at a ratio of 1:1 or 1:1:1, respectively. Although the materials have advantages of lower cost, higher capacity, and superior thermal stability than LiCoO2, they show deteriorated high capacity characteristics and lower temperature characteristics due to lower electronic conductivity than LiCoO2. In addition, even though the capacity is higher than that of LiCoO2, the energy density of the battery including the same is not improved due to its low tap density. Particularly, as these materials have low electronic conductivity (J. of Power Sources, 112, 2002, 41-48), the high power characteristics thereof are inferior to those of LiCoO2 or LiMn2O4 when used in a hybrid power source for electric vehicles.
Li[Ni1/2Mn1/2]O2 and Li[Ni1/3CO1/3Mn1/3]O2 can be prepared by simultaneously precipitating two or three elements in an aqueous solution using a neutralization reaction to provide a hydroxide or an oxide precursor, mixing the precursor with lithium hydroxide, and firing the same. Unlike the general co-precipitation reaction, the co-precipitated particle including manganese is shaped as an irregular plate and has a half tap density comparable to that of nickel or cobalt. For example, according to Japanese Patent laid-open Publication No. 2002-201028, a conventional reactor was used according to the inert precipitation process and the generated precipitate particles were widely distributed and each of the particles had a different primary particle shape. In addition, Japanese Patent laid-open Publication Nos. 2003-238165, 2003-203633, 2003-242976, 2003-197256, 2003-86182, 2003-68299, and 2003-59490, and Korean Patent Nos. 0557240 and 0548988, disclose a method of preparing a high capacity positive active material in which charge and discharge reversibility and thermal stability were improved by dissolving a nickel salt and a manganese salt, or a nickel salt, a manganese salt, and a cobalt salt in an aqueous solution, simultaneously introducing an alkali solution into a reactor while introducing reductants or an inert gas to provide a metal hydroxide or an oxide precursor, mixing the precursor with lithium hydroxide, and firing the same.
As described above, lithium transition element-based oxide having the R 3m layered crystal structure includes LiCoO2, LiNiO2, LiNi1−xCoxO2, LiNi1−x−yCOxMyO2 (M=Mn, Al, Mg, Ti, Ti1/2Mg1/2), LiNi1/3CO13Mn1/3O2, LiNi1/2Mn1/2O2, LiNixCO1−2xMnxO2, and Li1+z[NixCo1−2xMnx]1−zO2. Generally, such materials have a uniform metal composition between the particle surface and the body thereof. In order to provide excellent positive electrode performance, it is required to provide different functions to each of the inside and the surface of positive electrode powder particles. In other words, the composition of the inside of the particles has a lot of space for intercalating/deintercalating lithium ions, and the structure is stable as long as reaction with the electrolyte on the surface is minimized.
Proposals for changing the surface composition of the positive active material include a surface coating treatment. Such surface treatment includes coating the surface in a small amount of 1-2 wt % or less based on the total weight of the positive active material to provide a nanometer-thin coating layer, and heating the same after the coating step to provide a solid solution on the surface of the powder particle so that the metal composition thereof is different from that of the inside of the particle (J. Cho et al., J. Electrochem. Soc., 149, A127, 200; J. Dahn et al., Electrochem. and Solid State Lett., 6, A221 2003, U.S. Pat. No. 6,555,269, U.S. Pat. No. 6,274,273). When the coating layer is formed on the surface by coating and heating, the surface layer bound with the coating material has a depth of several tens or less nanometers and is different from the particle body. Thereby, it causes problems in that the coating efficiency is deteriorated after repeating several cycles. Further, the coating is not uniformly distributed on the surface so that the efficiency of coating is reduced.
In order to overcome such problems, Korean Patent Laid-open Publication No. 2005-0083869 disclosed a lithium transition element having a concentration gradient of the metal composition. After synthesizing the inner material, it is covered with a different composition on the surface thereof to provide a two-layered material. Then, it is mixed with lithium salt and subjected to heat treatment. The inner material may include a commercially available lithium transition element oxide.
According to this method, the different metal compositions for the inner layer and the outer layer may be synthesized, and the metal composition is not continuously varied. By the heat treatment, it is possible to provide the metal composition with a gradual gradient, but the concentration gradient of the metal ion is rarely generated due to thermal diffusion at a temperature higher than 850° C. Further, the synthesized powder has a low tap density since the chelating agent of ammonia is not used, so it is not suitable for a positive active material for a lithium rechargeable battery. Further, according to the method, it is hard to control the lithium amount of the outer layer when the lithium transition element oxide is used for the inner material, so the reproducibility is deteriorated.
According to Japanese Patent No. 2002-001724, in order to improve the thermal stability and the cycle-life characteristics of the Ni-based positive active material, the composite positive active material Li1.02Ni0.65Mn0.35O2 of which the cycle-life characteristic and the thermal stability are excellent, but the conductivity and the discharge capacity are deteriorated, is mixed with Li1.02Ni0.7Co0.3O2 of which the conductivity and the discharge capacity are excellent, but the cycle-life characteristic and the thermal stability are deteriorated. The composite oxide improves the cycle-life characteristic by increasing the mixing ratio of the high stable composite oxide, and improves the high capacity characteristics by increasing the mixing ratio of the high conductivity composite oxide. However, a positive active material with both high capacity and excellent cycle-life characteristics has not been provided.